Liquid-phase oxidation of an aromatic alkyl to an aromatic carboxylic acid is a highly exothermic chemical reaction. Volatilizable aqueous acidic solvents are used to contain the reaction mixture and to dissipate the heat of reaction. The oxidation of aromatic alkyls in the liquid phase to form aromatic carboxylic acids is generally performed in a vented, well-mixed oxidation reactor, with a substantial portion of the heat generated by the exothermic oxidation reaction being removed by refluxing a portion of the aqueous solvent and aromatic alkyl contained within the reactor as the reaction mixture.
The materials vaporized as a result of the heat generated in the exothermic reaction, together with unreacted oxygen and other aqueous components that may be present, pass upwardly through the reactor and are withdrawn from the reactor at a point above the reaction mixture liquid level in the reactor. The vapors are passed upwardly and out of the reactor to an overhead reflux condenser system where the vaporized solvent, water and aromatic alkyl are condensed. The condensed materials, now at a temperature less than the reactor contents' temperature, are returned to the reactor by gravity. The noncondensible gases, carried along with the vaporized reactor material, are vented.
In operation, the reactor is fed by a liquid feed stream containing the aromatic alkyl, aqueous acidic solvent and an oxidation catalyst. An oxygen-containing gas is separately introduced into the reactor for oxidizing the aromatic alkyl to aromatic carboxylic acid in the presence of the catalyst.
The reaction mixture contained in the reactor includes small crystals suspension of the produced aromatic carboxylic acid. Since the reaction mixture contains solid-phase and liquid-phase components, as well as the continuously introduced oxygen-containing gas, vigorous stirring of the reactor contents is necessary to suspend solids and to obtain a high quality product. The vigorous stirring is costly in terms of power input and maintenance; and even with high agitator power input the mixing effectiveness of prior systems has not been as good as desired.
In one system for the production of an aromatic carboxylic acid, the reaction takes place in a vertically disposed elongated vessel having a substantially cylindrical side wall and having an agitator mounted for rotation within the vessel on a shaft situated at about the axis of the vessel. The agitator drives an upper mixing element, or impeller, in the form of a 4-blade disc turbine at an intermediate location on the shaft and a lowermost mixing element, or impeller, in the form of a 4-blade turbine with pitched blades at the lower end of the shaft.
The disc turbine has a disc diameter of 0.6 times the turbine diameter; and each of its 4 blades has a radial length equal to 0.25 times the turbine diameter.
The bottom pitched blade turbine has a clearance clearance reactor bottom of about 0.35 to 0.39 times the inside diameter of the reactor vessel.
The blade thickness in the pitched blade turbine is about 0.0052 times the diameter of the lower mixing element. The cross section of each blade is rectangular.
A portion of the aromatic carboxylic acid produced in the course of the oxidation reaction forms finely divided solid crystals, because the product is nearly insoluble at the conditions of the reaction mixture. Because these crystals are of relatively higher density than the liquid in the reactor, a portion thereof tends to settle to the bottom of the reactor when the stirring is inadequate. Such settling can result in a buildup of solids in the reactor bottom which results in reactor burns or reactor fouling. In addition, in the stirred reactors prior to this invention, the stirring creates separate circulation loops, or cells, within the lower and upper portions of the reactor with little mixing between them, producing zones of poor mixing or dead spots.
The present invention, on the other hand, provides improved mixing effectiveness in the reactor vessel whereby zones of poor mixing are eliminated and solid crystals are suspended uniformly. Solid particles tending to settle at the bottom of the reactor vessel are impelled at relatively lower impeller speeds and relatively lower energy costs. An aromatic carboxylic acid of higher purity may be obtained as a result.